Multi-component prosthesis with increased wall flexibility and segmented locking ridge to facilitate component assembly

ABSTRACT

A multi-component hip joint prosthesis is provided comprising an acetabular cup and a plastic bearing liner, each of which defines a segmented open shell with an inferior segment thereof removed to provide enhanced flexibility. The enhanced flexibility substantially facilitates the assembly and disassembly of the components. The liner and the cup are retained in their assembled condition by interengagement between a plurality of ridges and at least one groove on the respective components. The plurality of independent ridges enhances the flexibility of the ridges, thereby further minimizing the assembly forces required. Interengageable keys and keyways on the respective components may be provided to prevent relative rotation therebetween. The cup may comprise a removable tab to facilitate the insertion of a tool between the cup and the liner for prying the liner from the cup if necessary.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.205,315, filed Jun. 10, now U.S. Pat. No. 5,135 which is acontinuation-in-part of U.S. patent application Ser. No. 492,133, filedMay 6, 1983 now abandoned and entitled "MULTI-COMPONENT PROSTHESIS WITHINCREASED WALL FLEXIBILITY FACILITATING COMPONENT ASSEMBLY", Michael J.Pappas and Frederick F. Buechel, inventors.

BACKGROUND OF THE INVENTION

This invention relates generally to a new and improved joint prosthesis,and more particularly relates to an improved multi-component jointprosthesis, such as for example a multi-component hip joint prosthesis,wherein at least one of the components may be an open shell.

The conventional prior art total hip prosthesis typically includes ametal femoral head fixtured to the femur and a plastic, generallyultra-high molecular weight (UHMWPe), cup fixtured by cement to theacetabulum such as the type disclosed in U.S. Pat. No. 4,123,806 issuedNov. 7, 1978 to Amstutz et al. The prior apt also discloses the use ofhip prostheses including metal backed acetabular components in which theacetabular component consists of a metal cup with a plastic bearinginsert; examples of such prostheses are given in U.S. Pat. No. 3,840,904issued Oct. 15, 1979 to Tronzo and U.S. Pat. No. 3,903,549 issued Sep.9, 1975 to Deyerle. Metal backed acetabular hip prosthesis componentshave several advantages. The metal or rigid outer acetabular cupproduces a much more uniform stress distribution at the interfacebetween the cup and the acetabulum with lower peak forces or stressthereby improving the possibility of long term fixation. Rigidity of themetal outer acetabular cup also reduces distortion of the plastic linerimproving its sphericity and therefore contact with the metal femoralhead of the hip prosthesis, thereby improving conditions for wearresistance. Further, the use of a separate bearing insert allowsreplacement of the insert if the insert is damaged intraoperatively, orif the insert becomes excessively worn as a result of long term use, orif as a result of some problem revision is necessary which involves achange in the insert bearing. For example, in the event of revision froma surface replacement type hip prosthesis, which has a fairly largediameter head, to a stem type femoral prosthesis, which includes usuallya smaller diameter head, one may simply remove the bearing liner leavingthe metal acetabular cup affixed to bone, and replace the liner withanother liner or insert of appropriate size for the revised unit. Suchrevision can, therefore, be made without disturbing the acetabularfixation thereby preventing damage to the acetabular bone.

There is, however, a potential disadvantage associated with the use of arigid or metal acetabular cup. In the event that a load is applied nearthe rim of the acetabular cup, the elastic properties of the underlyingbone combined with the rigidity of the cup can produce a situation inwhich the opposite rim tends to be lifted off of its bony bed. Thistends to produce tensile loads on the cup and such tensile loads areundesirable in maintaining long term fixation.

In the prosthesis disclosed in the Tronzo patent, an interlock 26 andinterlocking groove 28 as described in FIGS. 2-8 of Tronzo are used toprevent rotation of the liner relative to the cup. This connection,however, is not resistant to a tensile withdrawal of the cup from theliner. Such withdrawal can occur as a result of traction forces due to alayer of liquid interposed between the femoral component and the plasticliner coupled with distraction of the femur from the acetabulum duringnormal activity. Such a situation can produce withdrawal of the plasticbearing from the acetabular cup producing dislocation of the prostheticcomponent.

The prosthesis described in the Deyerle patent uses an arcuate ridge 32which engages an arcuate slot 44 as shown in FIGS. 1, 5 and 6 of Deyerleto restrict relative rotation. The Deyerle device uses screws by makinguse of an annular liner (not identified by number) in conjunction with aretaining screw 30 to trap the liner in the cup. This design, however,experiences difficulty in liner removal because removal of the linerrequires removal of the screws from the bone which may produce damage tothe bone. Further, the use of such a connection resistant againsttensile loading requires the use of screws and many surgeons wouldprefer in some applications not to use screws for fixation.

Both the Tronzo and Deyerle prostheses use screws. Neither, however,provides the ability of the screws to change their angular orientationsignificantly to facilitate fixation. Further, neither provides screwsnear the inferior rim to minimize the possible lifting of the inferiorrim as a result of loads applied near the superior rim. Such loads arenormal in walking and may exceed eight times body weight in stairclimbing and descent. Since the angular orientation of the screw is notadjustable in Tronzo and Deyerle devices, these screw configurationscannot take maximum advantage of possible superior bone stock for screwimplantation. Further, Deyerle and Tronzo prostheses both make use ofeither screws or spikes for fixation. However, when such acetabularcomponents are used with cement, such spikes or screws may not benecessary for fixation and their use makes the operative procedure moredifficult and introduces additional damage to the bone.

U.S. Pat. No. 3,608,096 to Link discloses a prosthesis which uses arelieved face on the acetabular shell where a segment of the shell isremoved by means of an oblique cut (not identified by number) asdescribed in Column 2, lines 69-72 and Column 3, lines 1-3 to provide abetter approximation to the shape of the natural acetabulum so as toincrease clearance reducing possible impingement with the femur duringcertain kinds of activity as described in Column 3, lines 26-29.Further, the outside section 3 as shown in FIG. 1 of the Link patent iseccentric to the cavity 2 although the nature and reason for thiseccentricity are not described by Link. Although this oblique and simplerelief provides improvement in fit and clearance, still better fit andclearance can be provided by a somewhat more complex relief of theinferior face of the acetabular component.

In view of the above, it is an object of the subject invention toprovide a multi-component prosthesis that facilitates assembly anddisassembly of the components.

A further object of the subject invention is to provide amulti-component prosthesis that enables the components to be securelyretained in their assembled condition and also achieves easy assemblyand disassembly for all dimensional ranges of the components dictated bymanufacturing tolerances and environmental conditions.

Another object of the subject invention is to provide multi-componentprostheses that provide a smaller range of insertion forces for anygiven range of manufacturing and other size variations.

SUMMARY OF THE INVENTION

The subject invention is directed to a multi-component prosthesis thatis easy to assemble and/or disassemble and that is substantiallyinsensitive to dimensional variations in one or more components.

The prosthesis may comprise a first component having a plurality ofindependently deflectable resilient ridges which are selectivelyengageable with a second component of the prosthesis. The ability of theflexible ridges to deflect independently substantially reduces assemblyforces as compared to prostheses with a single integral ridge extendingover a comparable distance.

Assembly of the components of a multi-component prosthesis also may befacilitated by constructing at least one of the components as anon-symmetrical segmented open shell, whereby the segmenting of theshell increases the flexibility of the shell wall, which flexibility inturn facilitates component assembly. Such flexibility also enhances theengagement of retaining means for retaining components together uponassembly. Thus, the above described segmented resilient ridges and thenon-symmetrical segmented open shell construction both rely on increasedflexibility to achieve the previously stated objective.

Screws utilized to secure a prosthesis component to a bone may berecessed within the seat of a screw hole formed in the component forreceiving the screw. A preferred area contact is achieved between theunderside of the screw head and the seat of the screw hole by makingboth the underside of the screw head and the screw hole seat spherical.Additionally, a second component engaging the first component may beprovided with a recess overlying the screw hole. Thus, in the event thescrew head extends upwardly out of the screw seat the recessaccommodates the screw head.

As used in the context of the present invention, and as used in thespecification and appended claims, the term "open shell" defines a shellsegment produced by cutting a closed shell (i.e. a shell withoutopenings) with a single cutting plane or planar cutting surface cuttingthrough both the exterior and interior surfaces of the shell. The term"segmented open shell" defines a shell segment produced either bycutting an open shell by additional cutting surfaces, or by cutting aclosed shell by: a non-planar cutting surface, (e.g. a cylindricalcutting surface); more than one cutting plane; or more than one planarcutting surface. The generation of slots is not considered segmentationin the context of the present invention, since the generation of a slotis the equivalent of splitting the shell rather than removal of a shellsegment. Although slotting normally involves removal of a thin segment,such removal is incidental to function.

The prosthesis comprises a cup which is engageable with the bone and abearing liner which is snapped into engagement with the cup. The innersurface of the bearing liner defines a generally spherical articulationsurface which engages the head of corresponding prosthetic component.The cup and the bearing liner are both segmented open shells as definedabove.

The snapped engagement between the cup and the bearing liner may bedefined by a groove on the inner surface of the cup and a plurality ofseparately deflectable ridges on the bearing liner. The provision ofseparately deflectable ridges substantially facilitates theintraoperative assembly of the prosthetic device, but still provides asecure engagement of the bearing liner in the cup. The dimensions of thecomponents may be selected such that the ridges are in a deflectedcondition when the components are assembled to achieve a desirablysecure interengagement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatical illustration of a surface replacement hipjoint prosthesis embodying the present invention and showndiagrammatically as being fixtured by bone ingrowth to the hip bone andfemur.

FIG. 2 is a front elevational view of a metal acetabular cup of thepresent invention, FIG. 2A is a cross-sectional view taken in FIG. 2along the line 2A--2A and in the direction of the arrows, FIG. 2B is across-sectional view taken along the line 2B--2B in FIG. 2A and in thedirection of the arrows, and FIG. 2C is a partial cross-sectional viewillustrating the manner of assembly of the acetabular cup and bearingliner retaining means of the present invention.

FIG. 3 is a front elevational view of a bearing liner embodying thepresent invention; FIG. 3A is a cross-sectional view taken generallyalong the line 3A--3A in FIG. 3 and in the direction of the arrows, andFIG. 3B is a cross-sectional view of an alternate bearing linerembodiment of the present invention, similar to FIG. 3A but taken in adirection opposite to the arrows 3A--3A in FIG. 3.

FIG. 4 is a side elevational view of a screw of the present inventionand FIG. 4A is a left end view of the screw of FIG. 4.

FIG. 5 is an assembly view of the acetabular cup of the presentinvention and a bearing liner of the present invention, the bearingliner being an alternate embodiment of the liner of FIG. 3, and suchillustration showing the assembly as it would look in cross section weresuch cross-sectional assembly view to be taken along lines such as2A--2A in FIG. 2 and 3A--3A in FIG. 3.

FIG. 6 is a diagrammatical illustration illustrating the assembly of thefemoral cap to the resected head of a natural femur.

FIGS. 7A-7D are diagrammatical illustrations of a teaching of thepresent invention as to the increase in flexibility of the wall of ashell to facilitate assembly of prosthesis components.

FIGS. 8-10 are cross-sectional views of a further alternate embodimentof an acetabular cup and plastic bearing liner of the present invention.

FIGS. 11-13 are diagrammatical illustrations showing the advantages ofthe further alternate embodiment and illustrating the features causingthis embodiment to be the preferred embodiment.

FIGS. 14A and 14B are, respectively, partial cross-sectional views ofthe superior aspect of the acetabular cup and plastic bearing liner ofthe alternate embodiment of FIGS. 8-10.

FIG. 15B is a front elevational view of the acetabular cup of thefurther alternate embodiment.

FIG. 15A is a cross-sectional view taken generally along the line 15--15in FIG. 15B in the direction of the arrows.

FIG. 16B is a partial front elevational view of the superior aspect ofthe plastic bearing liner of the further alternate embodiment of thepresent invention.

FIG. 16A is a cross-sectional view taken generally along the line 16--16in FIG. 16B in the direction of the arrows.

FIGS. 17-20 are sequential views showing respective stages in theassembly of the further alternate embodiment acetabular cup and plasticbearing liner.

FIG. 21 is a front elevational view showing the interior of still afurther embodiment of an acetabular cup in accordance with the subjectmatter.

FIG. 22 is a cross-sectional view taken along line 22--22 in FIG. 21.

FIG. 23 is a cross-sectional view taken along line 23--23 in FIG. 22.

FIG. 24 is a cross-sectional view showing the peripheral ridge andgroove of the acetabular cup illustrated in FIGS. 21-23.

FIG. 25 is a rear elevational view showing the exterior of a plasticbearing liner for use with the acetabular cup shown in FIGS. 21-25.

FIG. 26 is a side elevational view of the liner shown in FIG. 25.

FIG. 27 is a cross-sectional view taken along line 27--27 in FIG. 25.

FIG. 28 is a cross-sectional view similar to FIG. 27 but showing a linerof greater radial thickness.

FIG. 29 is a cross-sectional view showing the ridge of the liner ofFIGS. 25-28 engaged in the groove of the cup shown in FIGS. 21-24.

FIG. 30 is a cross-sectional view combining the acetabular cup as shownin FIG. 22 and the plastic bearing liner as shown in FIG. 27.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a diagrammatical illustration ofa natural innominate or hip bone 10 including the ilium 11, the ischium12 and the pubis 13 and a natural femur 16 having a neck 17. Also showndiagrammatically is a hip joint prosthesis embodying the presentinvention and indicated by general numerical designation 18. The hipjoint prosthesis 18 as shown is of the type commonly referred to in theart as a surface replacement hip joint prosthesis. However, it will beunderstood that the present invention is not limited to the surfacereplacement type prosthesis, but has wide application to other types ofprostheses, such as other types of hip and shoulder prostheses. Theprosthesis 18 is shown diagrammatically as being implanted in orfixtured to the hip or pelvic bone 10, or more particularly to theportion of the hip bone providing the acetabulum, or acetabular socketand to the femur 16. The hip joint prosthesis 18 includes a metalacetabular cup 20, a plastic bearing liner or plastic acetabular cup 30and a metal femoral component or cap 40 of the surface replacement type.However, other femoral prostheses may be employed, such as thosecommonly referred to as the femoral stem type prostheses.

In the embodiment shown, the metal acetabular cup 20 is provided with aporous outer surface or coating 21 into which the hip bone 10 may growfor permanent fixation. The cup 20 is temporarily fixtured to the hipbone 10 by the use of metal screws 51, 52 (not shown) and 53. The screws51-53 are used primarily to establish temporary fixation of theacetabular cup 20 to the hip bone 10 during the time required forbiological bone or bony ingrowth to occur into the porous coating 21 toprovide permanent fixation or to provide augmentation to fixation by useof bone cement. In the preferred embodiment, the metal acetabular cup 20is temporarily fixtured to the hip bone by the three metal screws, withmetal screw 51 being screwed into the ilium 11, metal screw 52 (notshown) being screwed into the ischium 12, and metal screw 53 beingscrewed into the pubis 13.

The plastic bearing liner 30 is snapped into the metal acetabular cup20, in a manner described in detail below, and its inner surfaceprovides the acetabular articulation surface.

The metal femoral cap 40 is provided with a highly polished exteriorsurface 42 providing the femoral articulation surface. The metal femoralcap 40 also comprises a generally hollow interior having an interiorporous surface, indicated by general numerical designation 44, intowhich femur bone may grow to permanently fixture the femoral cap 40 tothe resected head 19 of the femur 16 as shown in FIG. 6. Alternatively,the porous surface 44 may be used to improve cement fixation. Thefemoral cap 40 further comprises a relatively smooth or polished metalstem 46 which is used primarily for alignment purposes and to providesome resistance against fracture of the femoral neck. However, therelatively smooth surface of the metal stem 46 is not a fixationsurface. Temporary fixation, during the time required for permanent boneingrowth fixation to occur, is provided by a press fit between theinterior surface 44 and the resected femur head 19 and a press fitbetween the stem 46 and a hole 47 (FIG. 6) drilled centrally in theresected femur head 19 and femur neck 18. The femoral cap 40 shown inFIG. 1 is the subject of co-pending application Ser. No. 830,208 by thesame inventors as the present invention.

Referring now to FIGS. 2, 2A, and 2B, it will be noted that theacetabular cup 20 is a substantially hemispherical cup having arelatively thin wall 22 defined by concentric outer and inner surfaces23 and 24, respectively, having a common center 25 (FIG. 2A) from whichspherical radii 26 and 27 are struck. The acetabular cup 20 issymmetrical about the plane through which section 2A--2A is taken, butis asymmetrical with respect to other planes. In particular, theacetabular cup 20, at its inferior portion indicated by generalnumerical designation 28, is relieved as may be best seen in FIG. 2A.

One embodiment of the acetabular cup 20 is shown in FIGS. 2-2B. Cup 20is a multiple segmented shell of revolution, i.e. a segmented openshell, having a generating axis F--F and being of uniform wall thicknessand having concentric inner and outer surfaces. The face 60 of the cup20 is a surface produced by the intersection of a first cutting planeA--A perpendicular to a plane of symmetry of the cup 20, with such planeof symmetry lying in the plane of the drawing. The first cutting planeor planar cutting surface A--A partially defining the face 60 of the cupis closer to a parallel plane B--B through the center of the spheredefining the outer and inner surfaces 23 and 24, respectively, of thecup. The cup 20 is segmented by an inferior surface defined by theintersection of a second cutting plane or planar cutting surface C--Cperpendicular to the plane of symmetry and to the first cutting planeA--A defining the face 60 of the cup. The cup 20 has a transitionsurface 29 (FIG. 2) defined by an intersection of the shell ofrevolution with a segment D--D of a cylindrical cutting surfaceperpendicular to the plane of symmetry and tangent to the first andsecond cutting surfaces or planes A--A and C--C defining the face andinferior surfaces, respectively. Segmentation of the acetabular cup 20by cutting planes or surfaces A--A, C--C and D--D causes the cup to be a"segmented open shell" as defined above.

A ridge 70 (lower portion of FIG. 2A) protrudes from the inferior aspectof the cup adjacent to the inner surface 24. An annular groove 64 cutinto the inner surface 24 and lies in a plane parallel to and adjacentthe first cutting plane A--A. The cup 20 may be provided at its superiorportion, FIG. 2, with a radially inwardly directed key 65 extendinginwardly a distance from the face 60 of the cup 20. Additionally, thecup 20 may be provided with a pair of generally opposed slots 61--61extending inwardly from the face 60 at the lateral or side portions ofthe cup 20 as shown in FIG. 2. Further, the cup 20 may be provided withthree apertures or screw holes 66, 67 and 68 each for receiving a metalscrew, such as for example the metal screw 190 of FIG. 4. The aperturesmay be provided with a recessed spherical seat 69 for accommodating anymisalignment with the spherical underside 92 of the screw head 94 ofFIG. 4 with the screw head remaining entirely within the recess.Further, to permit the screw inserted through one of the apertures 66-68to engage the best available bone accessible through the aperture, thespherical seats 69 may be made oblong as shown more clearly with regardto apertures 67 and 68 in FIG. 2.

Referring now to FIGS. 3, 3A and 3B, it will be noted that the plasticbearing liner 30 is a substantially hemispherical cup defined byeccentric inner surface 32 and outer surface 34 and is symmetrical aboutplane 3A--3A, but asymmetrical in other respects. It will be noted inFIG. 3A that the inner and outer spherical surfaces 32 and 34 havemutually displaced respective spherical centers 36 and 38 with thespherical radius 80 of the inner surface 32 being struck from thespherical center 36 and with the spherical radius 82 of the outerspherical surface 34 being struck from the spherical center 38. Theplastic bearing liner 30 is further provided with a face indicated bygeneral numerical designation 85, complementary in shape to the face 60of the acetabular cup 20. The plastic bearing liner 30 used with theacetabular cup 20 of FIGS. 2-2A is provided with a flat region 86, acurved or cylindrical region 88 and a flat region 87 causing the plasticbearing liner 30 to be relieved at its inferior portion indicated bygeneral numerical designation 89, in the same manner as the metalacetabular cup 20 of FIG. 2A, i.e. by being segmented by cutting planessuch as A--A, C--C and D--D of FIG. 2A. The segmentation including theinferior relief causes the plastic bearing liner 30 to be a "segmentedopen shell" as defined above.

The outer surface 34 of the plastic bearing liner 30, as may be bestseen in FIG. 3A, is provided with an interrupted annular ridge 90. Moreparticularly, the annular ridge is interrupted by the relieved inferiorportion 89 or by the circular or cylindrical portion 88 of the face 85.The eccentric inner and outer surfaces 32 and 34, respectively, providethe plastic bearing liner 30 with a wall 92, as may be best seen in FIG.3A, which wall is thicker at its superior portion 93 and which isthinner at its lateral portions 94--94 (FIG. 3) adjacent the inferiorportion 89 of the plastic bearing liner 30. This is done to provide agreater thickness of bearing, thereby allowing greater wear in thesuperior aspect where most wear occurs. This also produces a somewhatthinner sidewall laterally, anteriorly and posteriorly, therebyincreasing flexibility of the wall 92 in the region where the wall mustbe compressed to assemble the plastic bearing liner 30 into theacetabular cup 20. Further, as may be best seen in FIG. 3, the plasticbearing liner 30 may be provided at its superior portion 93 with aradially inwardly extending keyway 97 for receiving the key 65 of theacetabular cup 20, as shown in FIG. 2.

The outer surface 34 of the plastic bearing liner 30 may be providedwith a plurality of mutually displaced recesses 101, 102 and 103 foroverlying the apertures 66, 67 and 68 of the cup 20, shown in FIG. 2,and their spherical, and oblong spherical, seats 69 which help preventcontact between the plastic bearing liner 30 and the screw head wherethe screw is not properly seated. The interrupted annular ridge 90 iscomplementary to and closely matches the interrupted annular ridge 64 ofthe acetabular cup 20. Similarly, the outer surface 34 of the plasticbearing liner 30 closely matches the spherical inner surface 24 of theacetabular cup 20. The spherical inner surface 32 of the plastic bearingliner 30 provides the acetabular articulation surface.

The inferior flat surface 87 of plastic bearing liner 30 engages theridge 70 on the acetabular cup 20 restraining, in conjunction with key65 and keyway 97, axial rotation of plastic bearing liner 30 withrespect to acetabular cup 20 about generating axis F--F as shown in FIG.5.

A cross section of the partially assembled insert and acetabular cup inthe region of the interconnecting ridge and groove is shown in FIG. 2C.The ridge and groove faces 125 and 126, respectively, are substantiallyparallel to the faces 85 and 60 of the insert and cup, respectively. Themedial aspect of the ridge is at an angle relative to the face. Uponassembly, using inward forces with substantial force components parallelto the generating axis (F--F of FIG. 2A), the medial aspect of the ridge90 engages an edge 123 of the interior surface 24 of the acetabular cup.Such engagement produces forces with substantial lateral componentsgenerally perpendicular to the generating axis, such components tendingto compress the plastic bearing liner 30 or expand the acetabular cup20. The side walls of the plastic bearing liner 30 or cup 20 arerelatively laterally flexible due to the above described segmentation.Therefore, sufficient deformation of the plastic bearing liner 30 andcup 20 is easily produced. This deformation allows the ridge 90 to passthe inside region 124 of the cup 20 adjacent the groove 64, and allowsthe ridge 90 to engage the groove 64, thereby relieving the compressiveand expanding forces and allowing the components to assume theundeformed states.

Removal of the plastic bearing liner 30 from the acetabular cup 20 byuse of outward forces with substantial force components parallel to thegenerating axis (F--F of FIG. 2A) is extremely difficult since theengaging faces 125 and 126 on the liner and cup respectively areperpendicular to the generating axes and thus fail to produce anysignificant lateral forces compressing the plastic bearing liner 30 orexpanding the acetabular cup 20. Rather the forces will be essentiallyparallel to the generating axis. Therefore, due to the curved shape ofthe components, these forces will tend to expand the plastic bearingliner 30 and compress the cup 20 thereby locking the componentstogether. Disassembly can be carried out only by use of instrumentsdesigned to be inserted into the slots 61--61 (FIG. 2) to apply lateralforces compressing the plastic bearing liner 30 or expanding the cup 20.Due to the nature of the segmentation of these segmented open shellcomponents described, relatively low lateral forces are needed todisassemble the components. Such lateral forces cannot be produced inthese components in an implanted prosthesis. Even if such forces couldbe produced, they would be resisted by the head of the femoral component40 which prevents contraction of the plastic bearing liner 30 and by theacetabulum. Thus the plastic bearing liner 30 cannot be removed while itis in engagement with the femoral component 40, and dislocationresulting from distraction forces as the result of fluid between thefemoral component and the liner cannot occur. For the case of a metalacetabular component 20, which is very stiff compared to plastic, and aplastic bearing liner 30, dislocation of the hip is necessary to removethe plastic bearing liner 30.

The increased flexibility of the segmented open shells described abovecan be understood by examining FIGS. 7A-D. FIGS. 7A and B illustrate a"segmented open shell" in accordance with the present invention, whileFIGS. 7C and D illustrate "open shells" of the prior art type disclosedin U.S. Pat. No. 4,123,806 to Amstutz et al. noted above. It is seenfrom FIGS. 7A-B that removal of the inferior shell segment removes amajor lateral load supporting segment. In shells where the wallthickness t is substantially smaller than the half the radius r of theshell, such segmentation eliminates most of the circumferentialcompressive stiffness of the wall, thus making bending stiffnessdominate. Since for such shells bending stiffness is much less thancircumferential stiffness, flexibility of the wall of the "segmentedopen shell" is greatly increased. Increased flexibility results in asmaller variation in inserting forces for any given range ofmanufacturing and other size variations while still achieving a tightfit of components within the tolerance range.

With regard to the screw 190, which may be utilized in the presentinvention, and referring to FIG. 1, it will be noted that in addition tothe above-described screw structure the end of the screw 196 may beprovided with radially disposed slots, as shown in FIG. 4A, which slotsprovide self-cutting action so that a cap need not be used inconjunction with the screw. Preferably, the screw is made of a metalalloy the same or similar to that of the metal acetabular cup 20 and ithas been found to be preferable to provide several sized screws fordifferent fixation conditions. The threads 198 are preferably largesince they are for the purpose of screwing and holding into cancellousbone. Further, it has been found that the screw for fixation into theischium 12, FIG. 1, is somewhat shorter than the screws used forfixation in the ilium 11 and pubis 13. Of course, the sphericalunderside 192 of the screw head 194 matches the spherical seats 69 ofthe apertures 66, 67 and 68 of FIG. 2.

It may be seen from FIG. 5 that the screw 190 can be moved through anangle β about center 111 with the spherical underside 192 of screw 190remaining in full contact with spherical seat 69 as observed by themotion of the screw axis from line 112 to 114 at which point the shank199 contacts the edge 115 of hole 66 limiting further motion.

Before describing in detail the manner of assembly and disassembly ofthe metal acetabular cup 20 and the plastic bearing liner 30, and theassembly of the metal femoral cap 40 to the resected femur 16, a briefdescription of the surgical implantation or fixation technique ispresented. The acetabulum, or acetabulum socket or cavity provided bythe hip bone 10, FIG. 1, is reamed in an ordinary fashion as would bedone for the implantation of any acetabular hip cup, such being wellknown to those skilled in the art, particularly orthopaedic surgeonsskilled in the art. A trial component is then used to orient theacetabular cup to the acetabulum and is used to mark out the location ofthe holes to be drilled for accepting the screws 190. The metalacetabular cup 20 is provided with a slightly larger spherical outersurface 23 than the prepared acetabular cavity and the acetabular cup isthen anatomically aligned and pressed into place. The holes are thendrilled into the acetabulum through the apertures 66, 67 and 68, andscrews 51, 52 and 53 of FIG. 1, which screws may be the screw 190 ofFIG. 4, are then screwed into the hip bone providing the acetabularcavity to temporarily fixture the metal acetabular cup 20 to the hipbone. It will be understood that the spherical underside of the screwheads, e.g. spherical underside 192 of FIG. 4, engages the sphericalseats 69 of the acetabular cup apertures and since the shank 199 of(FIG. 4) the screw is made to be much smaller than the diameter of theapertures, this allows the head of the screws to be pivoted about apoint 111, FIG. 6, so that the screw may be oriented at an angle βrelative to the axis 112 of the aperture as shown in FIG. 5. Thus, itwill be understood, positioning variation is provided by the presentinvention, and this positioning configuration has been found to beimportant in seeking out the best available bone for the screw toengage. Furthermore, this positioning variation is accomplished whilemaintaining excellent contact between the screw head and its sphericalseats. The oblong spherical seats of the apertures, FIG. 2, allow forsimilar misalignment but also allow for a further change in the locationof the screw within the confines of the seat thereby again enhancingeffective utilization of existing bone stock and permitting a greaterchange in location of the screw within the aperture to permit the screwto engage the best available bone accessible through the aperture.

The plastic bearing liner 30 is now "snap-fitted" into place in thefixtured metal acetabular cup 20 in accordance with the teachings of thepresent invention. Spherically radially inwardly directed forces, suchas may be generated digitally and quite easily by the fingers of theoperating surgeon, are applied to the thinner wall regions 94--94 of theplastic bearing liner 30 to flex the lateral thinner wall portionsinwardly. This permits the interrupted annular ridge 90 of the plasticbearing liner 30 to be inserted into and engage the interrupted annulargroove 64 of the metal acetabular cup 20. Thus, the inwardly flexedplastic bearing liner 30, in combination with the respective relievedinferior portions 28 and 89 of the acetabular cup 20 and plastic bearingliner 30, readily permit the flexed plastic bearing liner 30 to beinserted or "snap-fitted" into the metal acetabular cup 20. The key 65of the acetabular cup 20 will be aligned with and received in thekey-way 97 at the superior portion of the plastic bearing liner 30.Additionally, the flat end portion of the wall 92 of the plastic bearingliner 30 will be aligned with and engaged by the flat projection orridge projecting inwardly at the inferior portion 28 of the metalacetabular cup 20. The key and key-way, and engaged flat surfaces,resist relative rotation between the plastic bearing liner 30 and themetal acetabular cup 20 upon torsional loads applied thereto duringarticulation of the hip joint. Further, it will be understood, as may bebetter seen in FIG. 5, the respective faces 60 and 85 of the acetabularcup 20 and plastic bearing liner 30 align, transversely, and present acommon face for the assembled cup 20 and plastic bearing liner 30 andwith the common relieved inferior portions thereof permitting a greaterrange of articulation of the femur bone without impingement with theplastic bearing liner 30 and cup 20, thereby further reducing thepossibility of impingement of the cup 20 and plastic bearing liner 30 bythe femur during joint articulation.

With the acetabular cup 20 and plastic bearing liner 30 now firmly inplace in the acetabulum, the head of the femur 16 is resected orprepared by suitable instruments to provide a resected head, as shown inFIG. 6, complementary to the interior surface 44 of the metal femoralcup 40. More particularly, the resected head 19 of the femur 16 isprepared to provide the resected head with a flat portion 141 continuinginto a spherical portion 142 continuing into a cylindrical portion 143.It will be understood that the outer dimensions of the resected femurhead 19 are made somewhat larger than the inner dimensions of the innersurface 44 of the metal femoral cap 40 and the centrally drilled holeprovided in the resected femur head 19 and femur neck 18 is madesomewhat smaller in diameter than the diameter of the femoral cap stem46. Thus, the femoral cap 40 may be press fitted onto the femur head 19with such press fitting providing temporary fixation of the femoral cap40 to the femur head 19. The hip joint is then reduced with the outerspherical surface 42 of the metal femoral cap 40 being received withinthe spherical inner surface 32 of the plastic bearing liner 30 for jointarticulation. The articulation between the polished outer sphericalsurface 42 of the metal femoral head 40 and the spherical inner surface32 of the plastic bearing liner 30 provide low friction, long wearingarticulation and, the relatively larger articulation surfaces associatedwith the surface replacement prosthesis provide greater potential forextended wear resistance than do the smaller articulating surfacesassociated with the conventional total hip prosthesis known to the priorart.

It will be further understood in accordance with the teachings of thepresent invention that the screws described above, providing temporaryfixation of the metal acetabular cup to the hip bone providing theacetabulum provide temporary fixation during the period of time requiredfor hip bone ingrowth and to the porous outer surfaces 23 of theacetabular cup 20 which bone ingrowth provides the permanent fixation ofthe metal acetabular cup 20 to the hip bone 10. Similarly, the press fitbetween the metal femoral cap 40 and the resected femur head 19 providestemporary fixation of the metal femoral cap 40 to the femur during thetime required for femur bone ingrowth at the resected head 19 to growinto the porous inner surface 44 of the femoral cap 40 to permanentlyfixture the femoral cap 40 to the resected femur head 19. It has beenfound that such respective temporary fixations are sufficient tomaintain the respective components fixtured during normal jointarticulation and during the time required for bone ingrowth andbiological permanent fixation.

In accordance with the further teachings of the present invention, theplastic bearing liner 30 may be readily and easily removed from themetal acetabular cup 20. Referring to FIG. 2, and as noted above, thelateral portions of the metal acetabular cup 20 are provided withgenerally opposed slots 66 into which a tool may be inserted andoperated readily by digital force supplied by the operating surgeon toagain flex the inner lateral regions of the plastic bearing liner walls94--94 inwardly to flex the plastic bearing liner inwardly to permit theinterrupted annular ridge 90 to be disengaged from the interruptedannular groove 64 and such engagement, in combination with therespective relieved inferior portions 28 and 89 of the cap and liner,permit the inwardly flexed plastic bearing liner 30 to be readilyremoved from the metal acetabular cup 20. It will be understood that inaccordance with the further teachings of the present invention, theslots 61--61 could be provided on the outer surfaces of the plasticbearing liner 30, at the same positions, or mating opposed slots couldbe provided in both the metal acetabular cup and the plastic bearingliner.

Referring again to the recesses 101, 102 and 103, provided in the outerspherical surface 34 of the plastic bearing liner cup 30, FIG. 3, itwill be understood that these recesses overlie the screw heads of thescrews 51, 52 and 53 temporarily fixturing the metal acetabular cup 20to the hip bone 10, and it will be understood that these recessesaccommodate these screw heads even during any misalignment or relocationof the screws within the apertures as described above. Further, and inaccordance with the teachings of the present invention, the plasticbearing liner portions providing the recesses engage the metal screwheads and prevent them from becoming unscrewed during jointarticulation.

Referring again to FIG. 3B, it will be noted that the alternate plasticbearing liner shown herein, with a smaller inside diameter than theplastic bearing liner of FIG. 3A, may be used in the event that theconventional metal femoral stem is used in the femur instead of themetal femoral cap 40 of the present invention, the spherical innersurface 132 of the plastic bearing liner alternate embodiment 130 ofFIG. 3B being dimensioned to closely match the exterior surface of suchmetal femoral stem.

Still further in accordance with the teachings of the present invention,the bearing liner, instead of being plastic as described above, may beceramic and in such alternate embodiment it will be understood that therelatively thin walls of the metal acetabular cup 20 will be flexedoutwardly to permit the ceramic bearing liner to be inserted into andremoved from the metal acetabular cup 20; it will be understood that theceramic bearing liner is brittle and less flexible than the metalacetabular cup but will still undergo some slight inward flexing but dueto the relieved inferior portions only a very slight outward flexing ofthe metal acetabular cup or very slight inward flexing of the ceramicliner, in combination with the relieved inferior portions, will readilypermit the insertion and removal of a ceramic bearing liner from themetal acetabular cup.

An important advantage of a replaceable bearing liner is that a surfacereplacement type hip may be revised to a conventional total hip using afemoral stem without disturbing acetabular fixation by removing theplastic bearing liner shown as embodiment 30 intended for use with asurface replacement femoral component and replacing it with the plasticbearing liner embodiment 130 intended for use with a femoral stemprosthesis.

Referring again to FIGS. 7A-D where it is shown that by open shellsegmentation the flexibility of the side walls may be increased, itshould be observed that this increase in flexibility will result in anincrease in the amount of allowable engagement between the groove 64(FIG. 2A) and the ridge 90 (FIG. 3A) for a given assembly load comparedto a non-segmented open shell since the segmented open shell is moreeasily compressed. Thus, the strength of this engagement can beincreased and/or manufacturing tolerances associated with thisengagement can be less critical than those associated with anon-segmented open shell.

Referring now to FIGS. 8-10, there is illustrated a further alternateembodiment of the present invention also embodied as a multi-componenthip joint prosthesis indicated by general numerical designation 218 andincluding a generally semi-hemispherical outer metal acetabular cup 220and a generally semi-hemispherical inner plastic bearing liner 230. Thecup and liner are illustrated in their assembled position in FIG. 8 andshown, respectively, in cross section in FIGS. 9 and 10 with FIGS. 9 and10 being similar to the cross-sectional views of FIGS. 2A and 3A,respectively, and it will be understood that the front views of the cupand liner 220 and 230 merely would be similar to the front views shownin FIGS. 2 and 3, respectively, and hence are not shown.

Acetabular cup 220 and plastic bearing liner 230 are structurallysimilar to the earlier described acetabular cup 20 and plastic bearingliner 30, that is the metal acetabular cup 220 and the plastic bearingliner 230 are each a "segmented open shell" as defined hereinabove witheach having an inferior segment removed which increases the flexibilityof the cup and liner wall thereby facilitating their assembly anddisassembly by reducing the assembly and disassembly forces required. Itwill be recalled with regard to the "segmented open shell" of thepresent invention illustrated in FIGS. 7A and 7B and as taught in theassociated specification hereinabove, that were a major load supportinginferior segment resisting assembly forces not removed, but present asshown in the case of the prior art "open shells" illustrated in FIGS. 7Cand 7D, such inferior segment if present would resist the assembly (alsodisassembly) forces and would, as taught above, make the cup and linermore stiff in compression thereby requiring the application of greaterassembly and disassembly forces to produce assembly and disassembly. Inaddition, as also taught above, this is particularly advantageous withregard to intra-operative assembly of the prosthesis within a surgicalcavity.

Primarily, the hip joint prosthesis alternate embodiment 218 of thepresent invention differs from the earlier described embodiments due tothe fact that the metal acetabular cup 220 and plastic bearing liner 230are each provided with a protrusion, i.e. lateral protrusion, at theiranterior and posterior wall portions, the posterior protrusions 221 and231 of the cup 220 and the plastic bearing liner 230, respectively,being shown in the cross-sectional drawings of FIGS. 9 and 10.

Referring now particularly to FIG. 9, the segmenting of the segmentedopen shell acetabular cup 220 will be described in detail. FIG. 9 is across-sectional view, as noted above, and is taken through the plane ofsymmetry which plane of symmetry, it will be understood, lies in theplane of the drawing. The metal acetabular cup 220 is segmented by aplurality of cutting surfaces. In particular, a first cutting plane orplanar cutting surface A'--A' is perpendicular to the noted plane ofsymmetry. A second cutting plane or planar cutting surface C'--C' isperpendicular to the plane of symmetry and intersects the first planarcutting surface A'--A'. The second planar cutting surface C'--C' removesa major load supporting inferior segment resisting assembly anddisassembly forces as in the embodiment of the present inventionillustrated diagrammatically in FIGS. 7A and 7B, and relieves theinferior portion 228 of the cup wall 222. A third cutting plane orplanar cutting surface E--E is perpendicular to the plane of symmetryand inclined at an angle O with respect to the first cutting surfaceA'--A'. A first cylindrical cutting surface D'--D' is perpendicular tothe plane of symmetry and tangent to the second and third planar cuttingsurfaces C'--C' and E--E, respectively. Finally, a second cylindricalcutting surface H--H is perpendicular to the plane of symmetry andtangent to the first and third planar cutting surfaces A'--A' and E--E.The first planar cutting plane A'--A' is relieved or displaced mediallyfrom the center of curvature 225 as shown by the double headed arrow 227extending between the cutting surface A'--A' and a parallel plane B'--B'extending through the center of curvature 225. It will be noted fromFIG. 9 that such cutting surfaces define the cup face indicated bygeneral numerical designation 260 and that the second through fifthcutting surfaces produce protrusions at the anterior (not shown) andposterior 221 portions of the wall 222 of the segmented open shell ormetal acetabular cup 220.

The segmenting of such cutting surfaces may also be understood in thecontext of the closed shell 240 shown partially and in dashed outline inFIG. 9. The closed shell 240 is a solid of revolution (including thespherical wall 222 of the segmented open shell 220), has a generatingaxis F'--F' and has spherical inner and outer surfaces 241 and 242(portions providing the spherical outer and inner surfaces 222 and 224,respectively, of cup 220). It will be noted and further understood thatthe above-described cutting surfaces, A'--A', etc., segment by passingor cutting through the wall and inner and outer spherical surfaces ofthe shell as is known by those skilled in the art in the segmenting ofany shell.

Referring now to FIG. 10, it will be understood that the plastic bearingliner 230, as noted above, is also a segmented open shell beingsegmented by cutting surfaces A'--A', C'--C', E--E, D'--D' and H--H, asshown in FIG. 10, and which cutting surfaces are identical, orsubstantially identical, to the correspondingly identified cuttingsurfaces of FIG. 9. The cutting surfaces of FIG. 10 also define theface, indicated by general numerical designation 285, of the plasticbearing liner 230, and hence it will be understood that the faces 260and 285 of the cup 220 and the plastic bearing liner 230, respectively,are complementary as may be noted from the assembly view of FIG. 8. Asmay be understood by reference to FIGS. 9 and 10, generally, thesuperior portion of the respective faces 260 and 285 is defined by theplanar cutting surface A'--A' and the inferior portion of the respectivefaces is defined by the remaining cutting surfaces. Further, cuttingsurfaces C'--C', D'--D', E--E and H--H also produce protrusions at theanterior (not shown) and posterior 231 portions of the wall 232 of theplastic bearing liner 230. The planar cutting surface C'--C' removes amajor load supporting inferior segment resisting assembly anddisassembly forces (same as in the embodiment of the present inventionillustrated in FIGS. 7A and 7B) and relieves the inferior portion 289 ofthe liner wall 232 thereby increasing, particularly, the flexibility ofthe anterior and posterior wall portions. This facilitates the insertionof the plastic bearing liner 230 into the cup 220 for assembly and theremoval of the plastic bearing liner 230 from the cup 220 fordisassembly. The liner 230 is provided with a spherical outer surface233 for engaging the spherical inner surface 224 of the cup 220, andwith a spherical inner surface 234 for articulating with the outerspherical surface of a spherical head such as the outer surface 42 ofthe femoral cap 40 of FIG. 1. The outer surface 233 and inner surface234 may be either concentric or eccentric as shown in FIGS. 3A and 3Band described above.

As may be noted from FIG. 8, the posterior protrusions of the cup 220and plastic bearing liner 230 of the alternate hip prosthesis embodiment218 extend laterally beyond the corresponding portions of the cup 20 andliner 30 of the first embodiment indicated in dashed outline in FIG. 8.The significance and further advantages of these protrusions areillustrated in FIGS. 11-13.

Referring now to FIGS. 11-13, the lateral portion of a natural hip bone10' having a natural acetabulum or natural acetabular cavity 250 isillustrated in cross section. Were the earlier described hip prosthesisembodiment 18 (FIG. 1) to be dimensioned so as to precisely fit or matchthe natural acetabular cavity 250, as illustrated in FIG. 11, a portionof the hip prosthesis 18 would protrude laterally beyond the borders ofthe natural acetabular cavity 250 as illustrated by the opposed arrowsshown. Alternatively, were the earlier described hip prosthesis 18 to bedimensioned so as not to protrude laterally beyond the border of thenatural acetabular cavity 250, then as shown by the opposed arrows inFIG. 12, this would leave, undesirably, a substantial portion of theinferior aspect of the natural acetabular cavity 250 and its naturalacetabular cartilage uncovered. However, as may be noted from FIG. 13,the alternate hip prosthesis embodiment 218 of the present inventioneliminates the prosthesis extrusion illustrated in FIG. 11 and at thesame time substantially covers the natural acetabular cavity 250 and itsnatural acetabular cartilage as shown. Accordingly, it will beunderstood that the alternate hip prosthesis embodiment 218 is thepreferred embodiment of the present invention since it has betterconformance to the shape of the natural acetabular cavity 250 and allowsreplacement of most of the surface of the natural acetabular cavity andacetabular cartilage without protrusion of the prosthesis beyond thebony borders of the natural acetabulum. Such protrusion (FIG. 11) isundesirable since such protrusion may produce impingement between theacetabular hip prosthesis and either natural bone or a femoralprosthesis component thereby producing undesirable shearing loads on theacetabular hip prosthesis causing loosening. Where, as in the alternatehip embodiment 218 illustrated in FIG. 13, the hip prosthesis 218 iskept entirely within the bony borders of the natural acetabulum, suchimpingement loading is avoided thereby reducing any tendency forloosening of the acetabular component due to such impingement.

It will be understood that the metal acetabular cup 220, similar to theearlier described metal acetabular cup 20, is also provided with aplurality of screw holes having recessed spherical seats for receivingmetal screws (e.g. metal screw 90 of FIG. 4) to temporarily fixture thecup 220 to the hip bone (e.g. hip bone 10 of FIG. 1). Two of such screwholes are shown in FIG. 9 and identified as 266 and 268. Also,similarly, the cup 220 and plastic bearing liner 230 are provided withmutually engageable retaining means for maintaining them together uponassembly, viz. respective mating annular groove 264 and annular ridge290 of complementary configuration as shown in FIGS. 9 and 10, as withthe annular groove 64 and annular ridge 90 of FIGS. 2A and 3A, theinferior aspect of the groove 264 and ridge 290 are interrupted orrelieved inferiorly by the cutting planes C'--C' of FIGS. 9 and 10which, in particular, increases the flexibility of the ridge 290 andpermits the liner 230 to be readily "snap-fitted" into the cup 220 bythe application of spherically inwardly directed assembly forcesgenerated digitally and quite easily by the fingers of the operatingorthopaedic surgeon.

Referring now to FIGS. 14-19, there is shown a further alternateembodiment of the present invention including an alternate metalacetabular cup 320 (e.g. FIG. 14A) and an alternate plastic bearingliner 330 (e.g. FIG. 14B). For convenience of presentation, only thecross-sectional views of the upper face or rim portions of the cup 320and liner 330 are shown because it will be understood that the cup 320and liner 330 are each a "segmented open shell" as defined above witheach having a major inferior load bearing segment resisting assembly anddisassembly forces removed to increase the flexibility of the walls ofthe components and facilitate their assembly and disassembly. Except forthe structural differences to be described, the cup 320 otherwise may besimilar to either the cup 20 (FIG. 2A) or 220 (FIG. 9) and the liner 320otherwise may be similar to the liner 30 (FIG. 3A), liner 130 (FIG. 3B)or liner 230 (FIG. 10).

As may be noted from FIG. 14A, the metal acetabular cup 320 in additionto the annular retaining groove 364 is provided with an annular recess321 extending inwardly from the cup face 360, also, the face 326 of theretaining groove 364 is provided with a lead in angle which in thepreferred embodiment is approximately 15°.

The thickness t, indicated by the opposed arrows in FIG. 14A, isdetermined primarily by considerations involving the thickness of thescrew head (e.g. screw head 194 of FIG. 5) upon the screw head beingfully recessed within its spherical seat (e.g. spherical seat 69 of FIG.5). The use of the annular recess 321 increases the height of the key365 (FIGS. 15A and 15B) by an amount h indicated by the opposed arrowsof FIG. 15A beyond the height of the key 65 of FIG. 2. This provides alarger key which is more effective in providing axial orientation of theplastic bearing liner, e.g. plastic bearing liner 334 of FIG. 14A,relative to the metal acetabular cup 320, in providing resistanceagainst rotation of the plastic bearing liner 334 in the metal cup 320which upon assembly tends to roll with respect to the metal acetabularcup due to the engaging inner spherical surface 324 of the cup 320 andthe outer spherical surface 334 of the plastic bearing liner 330, and inproviding visual clues to the operating orthopaedic surgeon to assist inaligning the cup and liner for assembly particularly within a surgicalcavity.

The alternate embodiment plastic bearing liner 330, FIG. 14B, differsfrom the prior embodiments by being provided with an interrupted annularflange 331 which is complementary in shape to and which fits into theinterrupted annular recess 321 (FIG. 14A) of the metal acetabular cup320. In addition, the ridge or locking ridge 390 of FIG. 14B in thisalternate embodiment differs from the prior embodiment ridge or lockingridges, e.g. ridge 90 (FIG. 3A) and ridge 290 (FIG. 10), by beingprovided with an undercut 391 for increasing the inward flexibility ofthe ridge 390 in bending. In addition, the face 396 of the locking ridge390 is inclined at the angle α to match the lead-in angle α of the face326 of the retaining groove 364 (FIG. 14A) formed in the metalacetabular cup 320. The plastic bearing liner 330 is provided with akeyway as shown in FIGS. 16A and 16B for receiving the key 365 (FIGS.15A and 15B) in the same manner that the keyways receive the key in thealternate embodiments except that in this embodiment the keyway 365 andkeyway are greater in height as mentioned above.

Upon assembly, FIG. 17, the superior aspect or portion of the plasticbearing liner 330 having the keyway 337 formed therein is first insertedinto the metal acetabular cup 320 with the key 365 and keyway 397aligned and with the face 396 of the locking ridge 390 engaging the backof the key 365; this partially traps the superior aspect or portion ofthe plastic bearing liner 330 in the metal cup 320. Now, either theanterior or posterior portion of the flange 331 of the plastic bearingliner 330 is inserted into the corresponding anterior or posteriorportion of the recess 321 of the metal acetabular cup 320. This isillustrated in FIG. 18 which shows the posterior portion of the flange331 engaging the posterior portion of the recess 321. Thus, at thispoint in the assembly, it will be understood that the flange 331 andrecess 321 upon engagement help stabilize the plastic bearing liner 330in the metal acetabular cup 320 against rotation in the direction of thearrows shown in FIG. 18 and prevents the above-noted rolling tendency ofthe spherical outer surface 334 of the liner 330 upon engaging thespherical inner surface 324 of the cup 320. At this point in theassembly, the opposite side of the plastic bearing liner insert 330,i.e. the anterior portion of the flange of the plastic bearing liner ifthe posterior portion is first engaged, or vice versa, is then"snap-fitted" into the recess 321 of the metal acetabular cup. Thisaction produces deformation in the plastic bearing liner 320 whichdeformation takes two forms. First the anterior and posterior portionsof the wall of the plastic bearing liner 330 flex inwardly due to theirincreased flexibility described above and secondly the locking ridge390, which is interrupted or relieved inferiorly as described above toenhance its inward flexibility, also flexes inwardly thus facilitatingassembly of the plastic bearing insert 330 with the metal acetabular cup320; a fully assembled plastic bearing liner and metal acetabular cupare shown in FIG. 20.

Referring to FIG. 19, the purpose of the lead-in angle α (FIGS. 14A and14B) will now be more fully understood in that it will be understoodthat this angle α is needed to allow the flexible locking ridge 390 tomove into the groove 364. Since the tip of the flexible locking ridge390 first will flex or bend in the direction of the arrow of FIG. 18,the use of the matching lead-in angles α will facilitate the bending orsnapping back of the flexible locking ridge 390 (in the directionopposite to the direction of the arrow (FIG. 18) and into the groove364. It will be understood that the lead-in angle α must not be so greatthat the approximate location of the flexure center C (FIG. 20) of theflexible locking ridge 390 lies above a line 399 drawn perpendicular tothe face 396 of the locking ridge 390.

As explained in detail above, the "segmented open shell" constructionfacilitates the assembly and disassembly of the acetabular cup and theplastic bearing liner by increasing the flexibility of the plasticbearing liner compared to those of nonsegmented configuration andthereby reducing the assembly and disassembly forces required. Incertain situations, such as where the walls of the plastic bearing linerare thick and therefore stiff despite segmentation of the liner, it maybe desirable to further facilitate the assembly and disassembly of themulti-component prosthesis. However, facilitating the assembly anddisassembly of the prosthesis should not lead to a loose fit of therespective components in their assembled condition. The seeminglyincompatible objectives are achieved by the embodiments of the metalacetabular cup 420 and the plastic bearing liner 430 illustrated inFIGS. 21-30.

The cup 420, as shown in FIGS. 21-24, is structurally similar to thecups 20, 220 and 320 described above. In particular, the cup 420 is ofsegmented open shell construction with an inferior load bearing segmentresisting assembly and disassembly forces removed to increase theflexibility of the walls. The cup 420 is defined by a plurality ofcutting planes or surfaces to define a cup face edge 460, which issubstantially similar to the cup faces 260 and 360 described in detailwith respect to the cups 220 and 320 respectively.

The cup 420 includes opposed outer and inner spherical surfaces 423 and424 respectively, and is further characterized by a plurality of screwholes 466-468 through which metal screws, such as the metal screw 90 ofFIG. 4, may extend to temporarily fixture the cup 420 to the hip bone,such as the hip bone 10 shown in FIG. 1. As noted with the previouslydescribed embodiments, the screw holes 466-468 comprise recessedspherical seats 469 to enable the secure seating of the screw head at aplurality of different angles in accordance with the location of themost desirable bone stock in the patient.

The cup 420, as shown in FIGS. 21 and 22, includes a pair of opposedkeyways 427 and 428 which are disposed for receiving complementary keys437 and 438 on the plastic bearing liner 430 which with the key 442 inthe cup and keyway 456 in the plastic bearing liner align the cup andliner and prevent relative rotation between the cup 420 and plasticbearing liner 430 in response to torsional load created duringarticulation of the hip joint.

As with the previously described embodiment, the cup 420 ischaracterized by an annular recess 421 and an annular retaining groove464 which extend generally parallel to a planar portion of the cup faceedge 460. The retaining groove 464 is defined in part by a face 426which is aligned at a lead-in angle of approximately 45°, as shown mostclearly in FIG. 24. This lead-in angle of approximately 45° is greaterthan the lead-in angle of approximately 15° as defined by the face 326in the previous cup embodiment 320 depicted in FIG. 14A. The largerlead-in angle shown in FIG. 24 substantially conforms to the shape ofthe ridges on the plastic bearing liner 430, as explained further belowto prevent relative movement between the cup 420 and the plastic bearingliner in their assembled condition.

To disassemble the plastic bearing liner 420 from the cup 430, a sharplever-type tool 500 can be inserted into the small gap 445 shown in FIG.29 intermediate the inferior surface 444 of the key 442 of cup 420 andthe bottom 446 of the keyway 456 of plastic bearing liner 430 to pry theplastic bearing liner 430 out of its seated position.

Turning to FIGS. 25-29, the plastic bearing liner 430 includes inner andouter spherical surfaces 432 and 434 respectively with the outerspherical surface 434 substantially conforming to the size and shape ofthe inner cylindrical surface 424 of the cup 420. The inner sphericalsurface 432 of the plastic bearing liner 430 is selected to conform tothe size and shape of the spherical head on the femoral component of theprosthesis used therewith. In this regard, FIG. 27 shows a plasticbearing liner 430 for use with a femoral component having a relativelylarge head. FIG. 28 on the other hand shows a plastic bearing liner 430'with an outer spherical surface 434 dimensioned to mate with the innerspherical surface 424 of the cup 420, but with an inner sphericalsurface 432 dimensioned to mate with a femoral component having arelatively smaller radius head.

Returning to FIGS. 25 and 26, the plastic bearing liner 430 is of thesame segmented open shell configuration described with respect to thecup 420 and the previously described plastic bearing liners 230 and 330.The plastic bearing liner 430 comprises an interrupted annular flange431 which is similar to the flange 331 of the previously describedembodiment and which is dimensioned to be received within the annularrecess 421 of the cup 420. Similarly, the annular flange 431 isinterrupted by a keyway 456 disposed to be in line with the key 442 onthe cup 420 producing access to the gap 445 into which a sharp tool 500may be inserted to separate the liner 430 from the cup 420. The annularflange 431 is further interrupted by the segmented open shellconstruction of the liner 430 as explained in detail above.

The liner 430 is characterized by keys 437 and 438 which are disposedand dimensioned to be received in the keyways 427 and 428 of the cup420. As noted above, the interengagement between the keys 437, 438 and442 and the corresponding keyways 427, 428 and 456 align the cup andliner prevent relative rotation between the liner 430 and the cup 420 inresponse to torsional forces created by articulation of the hip joint.

The liner 430 is further characterized by a plurality of independentresiliently deflectable locking ridges 490a, 490b and 490c extendinggenerally around the periphery of the liner 430 and generally parallelto the interrupted annular flange 431. As shown in the broken lines ofFIG. 25, the adjacent but separate locking ridges 490a-b and 490b-c maybe separated from one another by a slot which permits each locking ridge490a, 490b and 490c to deflect independently of one another. However, analternate embodiment as shown in the solid lines provides substantialangular spacing between the adjacent locking ridges 490a, 490b and 490c.The separation between the locking ridges 490a-c enables each ridge todeflect independent of the others, reducing the forces needed to deflecteach ridge and thereby greatly facilitating the bending of the lockingridges 490a-c during both assembly and disassembly of the liner 430 andthe cup 420. In the particularly preferred embodiments, each lockingridge 490a-c extends through an angle of between approximately 30° and60°, with the preferred angle being between approximately 40° and 45°.The particular angle for each locking ridge 490a-c would be selected inaccordance with the relative preferred assembly and disassembly forces,which in turn would depend upon the particular materials employed forthe cup 420 and liner 430 and upon the relative dimensions. The ridgeconstruction indicated in the solid lines of FIG. 25 increases the ridgeflexibility by both permitting the ridges to function independently andby reducing the total length of the ridges that must be deformed duringthe installation process, thereby reducing assembly forces andfacilitating assembly. The same principles apply during disassembly, andthus the separate ridges 490a-c shown in FIG. 25 substantiallyfacilitate removing the liner 430 from the cup 420.

Although the cup 420 and liner 430 depicted in FIGS. 21-30 can be easilyassembled and disassembled, it is desirable to prevent relative movementtherebetween. The embodiment depicted in these latter Figures achievethis objective with an interference fit obtained by providing an outsideradius on the undeflected locking ridge 490a-c which exceeds the insideradius of the annular groove 464 by approximately 0.002-0.008 inch forall cups 420 and plastic bearing liners within the specified tolerances.Thus, for the largest cup 420 within the specified tolerances and forthe smallest plastic bearing liner 430 within the specified tolerances,there will still be deflection of the ridges 490a-c in the fully seatedcondition of the cup 420 and plastic bearing liner 430. As a specificexample, the groove 464 of the cup 420 may define an inner diameter of1.594±0.003 inch, while the outer diameter defined by the ridges 490a-cof the plastic bearing liner 430 may be 1.604±0.004 inch. Thus, thelocking ridges will be inwardly deflected by between approximately 0.1%and 0.4% of the diameter of the bearing liner upon assembly of the cupand bearing liner. Thus, as shown most clearly in FIG. 30, the ridges490a-c are maintained in an inwardly deflected state with appropriatebiasing forces being exerted against the radially outer base of groove464 of the cup 420. This interference fit with the radially outwardlydirected forces by the deflected ridges 490a-c substantially preventsrelative movement between the plastic bearing liner 430 and cup 420.

Movement between the plastic bearing liner 430 and the cup 420 isfurther prevented by aligning the ridge end 496 at an angle ofapproximately 45°, or substantially equal to the angle defined by theface 426 of the cup 420. By making these angles equal at approximately45°, a smaller clearance 445 therebetween is obtained, the ridge end 496and the face 426 are in face-to-face contact, and there is lesslikelihood of movement between the components in response to forcesparallel to the generating axis of the cup 420 and plastic bearing liner430.

The plastic bearing liner 430 is assembled into the cup 420 insubstantially the manner explained above. In this assembled condition,the interengagement of the keys 427, 428 and 442 in the keyways 437, 438and 456 respectively prevents relative rotational movement of theplastic bearing liner 430 and cup 420. The deflection and snappingengagement of the independent ridges 490a-c into the groove 464 greatlyfacilitates this assembly of components. However, the relative radialdimensions of the ridges 490a-c and the groove 464 maintain the ridges490a-c in a deflected condition when the plastic bearing liner 430 andcup 420 are assembled to prevent relative movement. It will beappreciated that the above described plastic bearing liner 430 and cup420 cooperate to simultaneously achieve the seemingly dichotomousobjectives of facilitating assembly and obtaining a more secureengagement. In particular, the segmentation of ridges 490a-c increasesflexibility of the ridges 490a-c and therefore facilitates assembly.However, this same flexibility due to the segmentation also enables theridges to be in a deflected condition in the groove 464 therebyachieving a secure engagement.

It will be understood by those skilled in the art, i.e. the jointprosthesis art, that the terms of reference used in the appended claimsand hereinabove, viz. anterior and posterior, inferior and superior,medial and lateral, and the orientation of the acetabular components andbearing liners as shown in the drawings, except for FIG. 1, are withreference to the left natural acetabulum as viewed facing the humanbody, and that such terms are merely terms of reference.

Further, it will be understood by those skilled in the art that theprosthesis described above is the subject of many variations andmodifications all within the scope of the present invention and that thepresent invention is limited only by the scope of the appended claims.For example, the above described independently deflectable segmentedridges need not be used with the segmented open shell since bothstructures can often achieve the same or similar objectives, and thusmay be employed independently.

What is claimed is:
 1. An orthopedic prosthesis comprising a firstcomponent having an exterior surface and a second component having aninterior surface, the interior surface being dimensioned and configuredto engage the exterior surface, one of said interior and exteriorsurfaces defining a ridge supporting surface, the other of said interiorand exterior surfaces defining a ridge engaging surface, a plurality ofelongate ridges, each said ridge having a first longitudinal sideintegral with the ridge supporting surface and a second longitudinalside spaced from the ridge supporting surface and opposed ends extendingbetween the respective sides, said ridges being in generally end-to-endrelationship with the ends of each ridge being spaced from the ends ofthe other of said ridges such that each of said ridge is independentlyresiliently deflectable, the ridge engaging surface comprising ridgeengaging means for engaging each of the elongate ridges for retainingsaid first and second components in interengaged relationship.
 2. Anorthopedic prosthesis as in claim 1, wherein the interior and exteriorsurfaces each are arcuate.
 3. An orthopedic prosthesis as in claim 2,wherein each of said ridges extends along a portion of the arcuate ridgesupporting surface defining an arc of between approximately 30°-60°. 4.An orthopedic prosthesis as in claim 1, wherein the ridge supportingsurface is the exterior surface of the first component.
 5. An orthopedicprosthesis comprising a first component having an arcuate exteriorsurface and a second component having an arcuate interior surface, theinterior surface of said second component being dimensioned andconfigured to engage the exterior surface of the first component, one ofthe exterior and interior surfaces defining a ridge supporting surfacecharacterized by a plurality of elongate ridges, each said ridge havingone elongate side unitary with the ridge supporting surface, a secondlongitudinal side spaced from the ridge supporting surface, and opposedends extending away from the ridge supporting surface and connecting therespective longitudinal sides, the ends of each ridge being spaced fromthe ends of each other of said ridges, such that each of said ridges isindependently resiliently deflectable relative to the ridge supportingsurface, the ridge engaging surface comprising means for engaging theridges of the first component for retaining the first and secondcomponents in engagement with one another.
 6. An orthopedic prosthesisas in claim 5, wherein the ridge supporting surface defines the exteriorsurface of the first component, and wherein the ridge engaging meansdefines a groove on the interior surface of the second component.
 7. Anorthopedic prosthesis as in claim 5, wherein each said ridge extendsthrough an angle of between 30°-60°.
 8. An orthopedic prosthesis as inclaim 7, wherein the ridges are disposed substantially in end-to-endrelation.
 9. An orthopedic prosthesis as in claim 5, wherein the ridgesare in a resiliently deflected condition when said first and secondcomponents are engaged.
 10. An orthopedic prosthesis comprising a cupdefining an open shell having a spherically generated interior surfaceand a liner defining an open shell having a spherically generatedexterior surface, the exterior surface of the liner being dimensionedfor engagement with the interior surface of the cup, the exteriorsurface of the liner being characterized by a plurality of elongatedridges, each said ridge having a first longitudinal side unitary withthe exterior surface of the liner and a second longitudinal side spacedfrom the exterior surface, the first sides of said ridges lyingsubstantially in a common plane each said ridge being independentlyresiliently deflectable relative to the exterior surface and relative toeach other of said ridges, the cup comprising means for engagement withthe ridges of the liner for retaining the cup and the liner ininterengaged relation with one another.
 11. An orthopedic prosthesis asin claim 10, wherein each said ridge includes opposed longitudinal ends,said ridges being disposed substantially in end-to-end relation with oneanother.
 12. An orthopedic prosthesis as in claim 10, wherein each saidridge extends through an arc on the spherically generated exteriorsurface of the liner, said arc being between approximately 30°-60°. 13.An orthopedic prosthesis comprising a cup defining an open shell havinga spherically generated interior surface and a liner defining an openshell having a spherically generated exterior surface, the linerincluding a generally planar edge adjacent the spherically generatedarcuate surface thereof, the exterior surface of the liner beingdimensioned for engagement with the interior surface of the cup, theexterior surface of the liner being characterized by a plurality ofelongated ridges, the ridges being substantially parallel to the planaredge of the liner, each said ridge having a first longitudinal sideunitary with the exterior surface of the liner and a second longitudinalside spaced from the exterior surface, each said ridge beingindependently resiliently deflectable relative to the exterior surfaceand relative to each other of said ridges, the cup comprising means forengagement with the ridges of the liner for retaining the cup and theliner in interengaged relation with one another.